Photochromic compounds typified by chromene compounds, fulgide compounds and spirooxazine compounds have a characteristic feature (photochromic properties) that they change their colors swiftly upon exposure to light including ultraviolet light such as sunlight or light from a mercury lamp and return to their original colors when they are put in the dark by stopping their exposure to light and are used for various purposes, especially optical materials, making use of this characteristic feature.
For example, photochromic spectacle lenses which are provided with photochromic properties by using a photochromic compound function as sunglasses which are quickly colored outdoors where they are irradiated with light including ultraviolet light such as sunlight and as ordinary transparent eyeglasses which are faded indoors where there is no irradiation, and demand for the photochromic spectacle lenses is growing nowadays.
To provide photochromic properties to an optical material, a photochromic compound is generally used in combination with a plastic material. Stated more specifically, the following means are known.    (a) A method in which a photochromic compound is dissolved in a compound and the compound is polymerized to directly mold an optical material such as a lens. This method is called “kneading method”.    (b) A method in which a resin layer containing a photochromic compound dispersed therein is formed on the surface of a plastic molded article such as a lens by coating or cast polymerization. This method is called “lamination method”.    (c) A method in which two optical sheets are bonded together by means of an adhesive layer formed of an adhesive resin containing a photochromic compound dispersed therein. This method is called “binder method”.
For optical materials such as optical articles provided with photochromic properties, the following properties are further required.    (I) The degree of coloration ata visible light range before ultraviolet light is applied (initial coloration) should be low.    (II) The degree of coloration upon exposure to ultraviolet light (color optical density) should be high.    (III) The speed from the stoppage of the application of ultraviolet light to the time when the material returns to its original state (fading speed) should be high.    (IV) The repeat durability of a reversible function between color development and fading should be high.    (V) Storage stability should be high.    (VI) The material should be easily molded into various shapes.    (VII) Photochromic properties should be provided without the degradation of mechanical strength.
For the manufacture of optical materials having photochromic properties by the above-described means (a) to (c), various proposals have been made to satisfy the above requirements. As for color optical density and fading speed, it is now desired that more excellent photochromic properties should be developed.
For example, the above-described kneading method has an advantage that photochromic plastic lenses can be mass-produced at a low cost by using glass molds, and most of photochromic plastic lenses are now manufactured by this method.
However, as strength is required for a lens substrate in the kneading method, it is necessary to enhance the mechanical strength of a matrix resin containing a photochromic compound dispersed therein. Therefore, it is difficult to develop excellent photochromic properties. That is, since the degree of freedom of the molecule of the photochromic compound existent in the matrix resin becomes low, a photochromic reversible reaction is impaired.
For example, as for this kneading method, WO2012/176439 and WO2014/084339 disclose a technique for adding a photochromic compound to a monomer composition comprising a polyisocyanate monomer and a poly(thi)ol monomer. Although photochromic lenses molded by polymerizing and curing these compositions have very high mechanical strength as they have a (thio)urethane bond, the degree of freedom of the photochromic compound is significantly reduced due to the existence of the hard segment of the (thio)urethane bond and therefore the lenses are unsatisfactory in terms of photochromic properties. WO2009/075388 discloses a photochromic curable composition comprising a specific (meth)acrylic polymerizable monomer composition and a photochromic compound. Although a photochromic lens molded by polymerizing and curing these compositions develops high photochromic properties while it has high mechanical strength, its mechanical strength is lower than that of a (thio)urethane lens and there is still room for the improvement of photochromic properties. Further, these (meth)acrylic lenses have a problem that it is difficult to improve their refractive indices.
Meanwhile, in the lamination method, as compared with the above-described kneading method, since photochromic properties are developed with a thin layer formed on the surface of a substrate, to develop the same color optical density as that of the kneading method, a photochromic compound must be dissolved in a high concentration. In this case, according to the type of a photochromic compound, there occurs a problem such as unsatisfactory solubility or precipitation during storage. Further, since the layer which develops photochromic properties is thin, the photochromic compound may be inferior in durability.
WO2011/125956 discloses that a photochromic curable composition is applied to a plastic lens by spin coating and optically cured to form a photochromic coating layer (this lamination method is also called “coating method”). Then, since photochromic properties are developed with a thin layer comprising a photochromic compound in all examples of WO2011/125956, when a photochromic compound having low solubility is used, color optical density tends to become low and further, the photochromic compound may be inferior in durability.
Thus, either one of color optical density and fading speed is apt to become unsatisfactory in the currently known technologies.